1. Technical Field
The present invention relates generally to a system and method for 3-dimensional (3D) image reconstruction in a spiral scan cone beam computed tomography (CT) imaging system and, more specifically, to a spiral scan cone beam CT system and method that computes a normalization correction for image reconstruction when a reduced pitch spiral scan is used to acquire cone beam projection data using a fixed size detector.
2. Description of Related Art
A system employing cone beam geometry has been developed for three-dimensional (3D) computed tomography (CT) imaging that comprises a cone beam x-ray source and a 2D area detector. A 2D area detector used for 3D imaging generally has detector elements arranged in a 2D array of rows and columns. An object to be imaged is scanned, preferably over a 360 degree angular range and along its entire length, by any one of various methods wherein the position of the area detector is fixed relative to the source, and relative rotational and translational movement between the source and object provides the scanning (irradiation of the object by radiation energy). The cone beam approach for 3D CT has the potential to achieve 3D imaging in both medical and industrial applications with improved speed, as well as improved dose utilization when compared with conventional 3D CT apparatus (i.e., a stack of slices approach obtained using parallel or fan beam x-rays).
As a result of the relative movement of the cone beam source to a plurality of source positions (i.e., xe2x80x9cviewsxe2x80x9d) along the scan path, the detector acquires a corresponding plurality of sequential sets of cone beam projection data (also referred to herein as cone beam data or projection data), each set of cone beam data being representative of x-ray attenuation caused by the object at a respective one of the source positions.
U.S. Pat. No. 5,390,112 entitled xe2x80x9cTHREE-DIMENSIONAL COMPUTERIZED TOMOGRAPHY SCANNING METHOD AND SYSTEM FOR IMAGING LARGE OBJECTS WITH SMALLER AREA DETECTORSxe2x80x9d, issued on Feb. 14, 1995 to Kwok Tam, and U.S. Pat. No. 5,463,666 entitled xe2x80x9cHELICAL AND CIRCLE SCAN REGION OF INTEREST COMPUTERIZED TOMOGRAPHYxe2x80x9d, which issued on Oct. 31, 1995, both of which are incorporated herein by reference, describe a spiral scan cone beam CT system in which an x-ray source following a spiral scan path is used to image a relatively long object, wherein the x-ray detector is much shorter than the object. The only height requirement for the detector is that it be longer than the distance between adjacent turns in the spiral scan of the x-ray source.
More specifically, FIG. 2 is an exemplary diagram of a scanning trajectory 20. Specifically, the source scanning trajectory 20 comprises a helical (spiral) path located on the surface of a predetermined geometric surface (such as a cylinder) radially centered on axis Z. The helical path 20 defines a plurality of stages 211, 222, . . . 22n that are mutually spaced and surrounding an object O (or a region of interest (ROI) portion of an object) under examination such that each plane passing through the object O intersects the scanning trajectory 20 in at least one point. The term stage refers to each of the turns or revolutions formed by the helical path about axis 12, for example.
As the cone beam source 14 follows the scan path 20, the detector 16 acquires many sets of cone beam projection data, each set representative of the x-ray attenuation caused by the object O at each of a plurality of source/detector positions along the scan path 20. The cone beam projection data is then manipulated to reconstruct a 3D image of the object using any suitable image reconstruction protocol.
It is known in the art that to ensure that the cone beam data set acquired via such scanning trajectory is complete, each plane passing through the object O should cut the scanning trajectory 20 in at least one point. For example, as shown in FIG. 3, any plane (such as plane 24) intersecting the object O must also intersect the boundary of the geometric surface which surrounds object O, being that the scanning trajectory 20 is defined upon such geometric surface. In the case illustrated in FIG. 3, the geometric surface corresponds to the surface of a cylinder 26 that surrounds the object O. In the exemplary diagram, the curve of intersection between plane 24 and cylinder 26 comprises an ellipse 28. Further, it is shown that the curve of intersection between plane 24 and the cylindrical object O is also an ellipse 30 which is enclosed by scanning ellipse 28. Therefore, it should be appreciated that since the scanning helical path lies on the surface of cylinder 26, then the scan path intersects plane 24 at points 321 . . . 32n that collectively lie on the boundary of the geometric surface upon which the helical path is defined, that is, such points of intersection lie on scanning ellipse 28.
Referring again to FIG. 2, the criterion that any plane intersect at least one point on the scan path generally assumes that the detector 16 is fixed relative to the source 14 and that the entire object can be scanned within the field of view of the source. As explained in the above-incorporated U.S. Pat. No. 5,390,112, for example, the foregoing criterion can be advantageously satisfied if the height dimension H of the detector 16 extends just sufficiently along a direction generally parallel to axis Z to span at least the two consecutive stages in the helical path having the largest spacing therebetween as represented by L, that is, the largest spacing between corresponding points of such consecutive stages along axis Z. In some applications, the spacing between successive stages could vary depending on the specific scanning implementation. Alternatively, the scanning trajectory may be comprised of stages wherein the spacing L (i.e., pitch) between any two successive stages along axis Z is substantially equidistant.
In a cone beam CT system, to achieve optimal performance and efficiency of the detector, an optimal spiral pitch L is selected based on the detector height H. Since the pitch is determined by the table translation speed, the fixed pitch means that the table translation speed is fixed for the cone beam CT system. Thus, an optimum spiral pitch is determined by the detector height H and therefore is not adjustable.
In some circumstances, however, it is desirable to increase the photon counts on the detector to enhance the S/N for the reconstructed image. Photon counts are increased by, e.g., maintaining the scan time, and decreasing the table translation speed, which in turn reduces the spiral pitch. If the pitch is smaller, since the detector size is fixed, only a center portion of the detected cone beam image contributes to image reconstruction, and the data at the top and bottom edges of the detector amount to unnecessary exposure. In other words, when the pitch of the spiral scan is reduced from the optimal pitch based on the detector geometry, the photon efficiency of the system is compromised.
Accordingly, a system and method that would provide efficient and accurate reconstruction of an image by reducing the pitch and thereby increasing the x-ray exposure to obtain higher signal-to-noise, while using the same detector, is highly desirable.
The present invention is directed to a method for 3D image reconstruction in a spiral scan cone beam computed tomography (CT) imaging system that allows the pitch of spiral scan projection to be reduced, thereby increasing the x-ray dosage to obtain a higher S/N (signal-to-noise) ratio, while achieving efficient use of the same detector. In addition, an image reconstruction protocol according to the present invention computes a correction factor for integration planes that intersect a 1/n (n=3, 5, 7, 9, etc) reduced-pitch spiral path (which surrounds a ROI) at a number of points M, where M less than n (n=1, 3, 5, 7, 9, etc.), in a 1/n reduced pitch spiral scan. Despite the reduced pitch, these M less than n integration planes do not provide increased flux despite (in contrast to integration planes that intersect the object and scan path in n or greater locations). Consequently, by calculating the contribution of the projected image data from the M less than n planes, and adding an appropriately computed correction factor, uniform image enhancement (e.g., higher S/N ratio) can be achieved across all integration planes by virtue of the reduced pitch and correction factor.
In one aspect of the present invention, a method for image reconstruction in a spiral scan imaging system comprises the steps of:
acquiring cone beam projection data for a 1/n reduced-pitch spiral scan of a ROI (region of interest) of an object, wherein the step of acquiring comprises applying radiation from a cone beam source towards the object from a plurality of source positions along the reduced-pitch spiral scan path that encircles the ROI, and acquiring a set of cone beam projection data on an area detector, the set of cone beam data corresponding to a respective portion of the ROI at each of said source positions;
identifying a set of integration planes that intersect the ROI and that intersect the reduced-pitch spiral scan path at M less than n locations;
computing a contribution of cone beam image data for the identified set of integration planes;
computing a correction factor for the identified set of integration planes and adding the correction factor to the cone beam image data associated with the identified set of integration planes so as to increase the S/N (signal-to-noise) ratio;
reconstructing an image of the ROI using, in part, the corrected cone beam image data.
In another aspect, the step of identifying a set of integration planes comprises the step of determining a set of parallel integration planes in a direction 2, wherein 2 is within an angular range determined by a first and second curved mask boundary of a data mask that is used to acquire the cone beam projection data. Preferably, the first curved mask boundary is defined by cone beam projections of at least two spiral scan path turns above a current source position, and wherein the second curved mask boundary is defined by cone beam projections of at least two spiral scan path turns below the current source position.
In another aspect, the angular range is determined by a pair of common tangents in the direction of angles xcex8t(∞) and xcex8t(xe2x88x92∞), respectively, which diagonally connect the first curved mask boundary and the second curved mask boundary, wherein the angle xcex8t(∞)=tanxe2x88x921(P/2xcfx80R) is the direction of the projection of the scan path direction, wherein P denotes a pitch of the scan path and R denotes the scanning radius.
In yet another aspect, the step of computing a contribution of cone beam image data for the identified set of integration planes comprises the step of computing image data along a plurality of lines in the cone beam data in an angular range R=[xcex8t(xe2x88x92∞)xe2x88x92(xcfx80/2), xcex8t(∞)+(xcfx80/2)] using a filtered backprojection method.
In another aspect, the step of computing a contribution of cone beam image data for the identified set of integration planes comprises the step of computing image data along a first and second parallel line in an angular range R=[xcex8t(xe2x88x92∞)xe2x88x92(xcfx80/2), xcex8t(∞)+(xcfx80/2)] using a Hilbert Transform, wherein the first parallel line is tangent to the first curved mask boundary and the second parallel line is tangent to the second curved mask boundary.
In yet another aspect, the correction factor comprises a multiplying the contribution of the cone beam image data associated with the identified set of integration planes by a factor that corresponds to the amount of pitch reduction.
These and other objects, features and advantages of the present invention will be described or become apparent from the following detailed description of preferred embodiments, which is to be read in connection with the accompanying drawings.